1. Field of the Invention
The present invention pertains to optical modulators. Moreover, the present invention pertains to operation and construction of such modulators, and the use of such modulators in optical media recording devices.
2. Description of the Related Art
In an electro-optic modulator, the index of refraction of the electro-optic material changes with an externally applied voltage along the direction of the electric field. Thus, when the external electric field is on, the index of refraction along the field lines has a different value than the index of refraction perpendicular to the field lines. In a transverse electro-optic modulator, a light beam traverses the modulator in a direction perpendicular to the external electric field while its polarization is both planar and is at 45 degrees to the external electric field. Inside the electro-optic material, the light beam polarization is broken into two components along the two indices of refraction. The polarization component that is parallel to the external electric field will traverse the length of the electro-optic material either faster or slower than the other orthogonal polarization component depending on the sign of the index of refraction change. If the modulated index of refraction is smaller than the unmodulated one, then the polarization component along the modulated axis advances relative to the unmodulated polarization axis. The modulation voltage that causes a 180 degrees phase difference between the two orthogonal polarizations of the light beam is called the pi voltage (Vxcfx80). At Vxcfx80, the modulator acts as a half-wave retarder. In other words, an incident light beam polarization at 45 degrees to the external electric field will be rotated by 90 degrees from its initial orientation upon exiting the electro-optic modulator. The modulator becomes an on/off switchable optical device by placing a polarizer after the electro-optic modulator with its polarization axis 90 degrees to the initial orientation of the light beam polarization. With the modulator in the xe2x80x9coffxe2x80x9d position (no voltage applied) no light passes through the polarizer. With the modulator in the xe2x80x9conxe2x80x9d position (Vxcfx80 applied), the light beam passes through the polarizer. These materials have other properties: they have a very large dielectric constant, they can be electrically lossy, and they can exhibit some charge screening.
In a transverse electro-optic modulator, one electrode is typically driven to a high voltage at a low duty cycle and the other electrode is grounded. The electric field at which the phases of orthogonal polarizations vary by half a wavelength is called E(pi) and is calculated by dividing Vn by the distance between the electrodes.
Some applications require 100% optical response, and drive the modulator between 0 and +E(pi). For example, as shown in FIG. 1, a typical modulator drive range for such applications is between 0 and +E(pi).
An electro-optic modulator is driven between discrete operating points such that a reversal of applied field polarity reduces harmful charge screening effects and the voltage swing between the operating points is reduced from previously used voltage swings.